More Evidence That Sleep Helps Strengthen Memory

Sleep may aid in the reinforcement of memory, a new study in rats suggests.

Researchers found that rats' memory of a particular odor was strengthened when they were exposed to that odor during slow-wave sleep, compared with when they were exposed to the odor during wakefulness.

"We know that during slow-wave sleep, the brain's sensory systems are far less responsive to normal inputs," study researcher Donald Wilson, Ph.D. of the NYU Langone Medical Center and Nathan Kline Institute for Psychiatric Research said in a statement. "Our data suggest this sensory isolation may help allow replay of learned information in the absence of external interference, providing strong, precise memory of important information."

The rats in the Journal of Neuroscience study were initially conditioned to recognize a particular odor by being administered a mild foot shock during wakefulness in response to the odor information; the rats' fear response to the odor was used as a gauge of their memory of the odor.

Then, researchers exposed the rats to that odor while they were sleeping, particularly during the slow-wave phase of sleep; instead of actually having the rats smell the odor, they electrically stimulated brain circuits linked to odor processing in order to control the rats' odor experience.

The researchers found that when the rats were exposed to the odor during the slow-wave sleep, their memory of that odor was enhanced compared with rats who had the odor replayed when they were awake or who never had the odor replayed at all.

"Our findings confirm the importance of brain activity during sleep for both memory strength and accuracy," Wilson said in a statement. "What we think is happening is that during slow-wave sleep, neurons in the brain communicate with each other, and in doing so, strengthen their connections, permitting storage of specific information."

This isn't the first time research has shown that sleep can help to strengthen memories. A small study in the journal Science showed, for instance, that people performed better on a test of memory after taking a quick nap, though that study did not show that you can learn new things during sleep -- only that existing memories can be strengthened.